1. Field of the Invention
The present invention is related to cellular devices, systems and methods, and more particularly, to cellular devices, systems and methods using macro-diversity and dynamic channel allocation.
2. Description of the Related Art
Cellular telephone systems typically use several cells to serve a service area. Each cell has a base station capable of establishing radio connections with mobile units in the cell. The channels used to establish the radio connections can be implemented as a frequency division multiple access (FDMA) system in which a particular frequency carries a respective channel. Alternatively, a channel can be implemented in a time division multiple access (TDMA) channel in which a channel has a particular frequency and time slot. Code division multiple access (CDMA) systems are also used to establish channels, but these CDMA systems are not relevant to the subject invention.
The channels can be allocated to a cell in a fixed or dynamic manner. In fixed channel allocation, a cell is assigned a fixed set of channels to use for communications with mobile units in that cell. The fixed set of channels is determined to ensure that other nearby cells are not using a channel in the fixed set to avoid unacceptable levels of cochannel interference (that is, interference caused by more than one cell attempting to communicate simultaneously on the same channel). In dynamic channel allocation, a cell can use as many of the available channels as needed to serve the calls for that cell, as long as the use of any allocated channel will not cause unacceptable levels of cochannel interference from other nearby cells. Thus, because dynamic channel allocation allows a cell to adjust the number of channels used by that cell depending upon the number of calls to be served, dynamic channel allocation increases the trunking efficiency of a cellular system relative to fixed channel allocation.
Whether a conventional cellular system uses fixed or dynamic channel allocation, as a mobile unit with an established radio connection moves across a boundary between two cells, the original channel used to communicate with the mobile unit must be changed to a new channel used in the cell entered by the mobile unit. This operation of changing the channel used to communicate with the mobile unit on an established connection, is called a `handoff`.
One problem that occurs as a mobile unit moves along the boundary between two or more cells, is that multiple handoffs can occur. The resulting processing load on the cellular system resulting from multiple handoffs can be significant. This problem is commonly known as the `ping-pong` effect.
Another problem which has been identified in cellular systems, is the temporary interruption of a radio connection by a building or terrain, for example, interceding between a mobile unit and a base station serving the cell. One solution to this interruption of communication known as `shadowing` is to provide more than one communication path to serve a call, a technique termed `macro-diversity`. U.S. Pat. No. 4,932,049 applies macro-diversity within a cell (i.e., intracell macro-diversity) by using more than one base station arranged at different locations within the cell, to communicate with a mobile unit. However, because base stations are extremely expensive, using more than one base station for a cell can be an uneconomical and impractical solution to shadowing.
Another consideration important to a cellular telephone system, is the desirability of increasing the capacity of the system. To increase the capacity, a channel can be reused within a certain distance of a cell using the same channel as long as cochannel interference is below an acceptable level. This distance is called a `reuse distance` and is usually defined as a distance at which a signal-to-interference ratio (SIR) is above a specified value.
The reuse distance can be decreased by using directional (rather than omnidirectional) antennas at base stations serving cells in a cellular telephone system. In this situation, a cell is divided into sectors corresponding to respective directional antennas. Because the directional antenna has a relatively limited transmission area, the same channel can be reused relatively nearby the directional antenna. Accordingly, the reuse distance is reduced and the capacity is correspondingly increased, in cellular telephone systems using sectoring.
Although the use of sectoring increases the capacity of a cellular telephone system, the use of sectoring also increases handoff processing because handoffs between channels are needed even as a mobile unit moves across a sector boundary. Also, the ping-pong problem is an even greater concern if sectoring is used, because there are more sector boundaries than cell boundaries, so ping-pong effects are likely to occur more frequently in cellular systems using sectoring.